1. Field of the Invention
The present invention relates to a method for monitoring a semiconductor wafer, and more particularly, to a method for monitoring a semiconductor wafer in a chemical mechanical polishing process.
2. Description of the Prior Art
In semiconductor processing, chemical-mechanical polishing (CMP) is a planarization technique for uniformly removing a deposited layer from the surface of a semiconductor wafer. When performing the CMP process, proper monitoring is essential to avoid any manufacturing or polishing errors that may affect the yield. In the prior art, a CMP end-point detection system uses an optical theorem to monitor the dielectric layer to decide when to stop polishing, i.e., the end-point of polishing. However, the prior art monitoring method is not continuous. It lacks the ability to know, at any given time, if the polishing process is complete or to standard. Consequently, if any mistakes occur before the end-point of the polishing is reached, there is insufficient time to stop the process.
In the prior art, a supporting stand is used during the CMP process. This stand comprises a pedestal for supporting the semiconductor wafer and a polishing pad installed on the pedestal for polishing the surface of the semiconductor wafer. There is a hole in the polishing pad. The stand is connected to a liquid transmitting system. The liquid transmitting system is responsible for directing the necessary polishing slurry to the stand for use in the CMP process. During CMP processing of the deposited layer, such as a dielectric layer, the semiconductor wafer is first positioned horizontally on the pedestal. The pedestal then spins at a certain speed. The polishing slurry is uniformly sprayed onto the surface of the spinning semiconductor wafer and undergoes chemical reactions with the dielectric layer. At the same time, the polishing pad above the pedestal presses downward upon the surface of the semiconductor wafer to perform a mechanical polishing. The chemical reaction of the polishing slurry in conjunction with the mechanical polishing of the polishing pad, and with the parameters of the process properly set, can remove the portion of the dielectric layer that lies on the surface of the semiconductor wafer. The end-point detection system determines the polishing end-point of the dielectric layer in the CMP process. This is accomplished by monitoring the reflected light beam from the dielectric layer via the hole in the polishing pad.
Please refer to FIG. 1. FIG. 1 is a graph of intensity versus time of the reflected light beam in a prior art method for monitoring a dielectric layer in a chemical mechanical polishing process. In the prior art method, the end-point detection system exposes the dielectric layer of the semiconductor wafer to an input light beam of fixed wavelength at a predetermined oblique angle to generate a reflected light beam that passes through the hole of the polishing pad when performing the CMP process. The intensity of the reflected light beam is detected. Measuring and recording the intensity of the reflected light beam continues at every time point. In FIG. 1, t2 in the figure denotes the predetermined point in time to stop the CMP process. The CMP process automatically stops at t2. However, as the polishing time increases, and before the time t2 is reached, the dielectric layer becomes thinner, which results in variations in the intensity of the reflected light beam. Such a change is depicted in the region between a time t1 and the predetermined end-point time t2, the region xcex94t2. When the surface of the semiconductor wafer becomes excessively polished to the extent that the layer below the dielectric layer is reached, the intensity of the reflected light beam changes sharply due to the differences in the properties of the different materials. In order to avoid over-polishing, the CMP process must stop as soon as the sharp change of the intensity of the reflected light beam is discovered. So, in the prior art method of monitoring a semiconductor wafer in a chemical mechanical polishing end-point detection system, changes of the intensity of the reflected light beam within the time period xcex94t2 are detected so as to determine the end-point of polishing of the dielectric layer and to monitor the accuracy of the CMP process.
The end-point detection system for a dielectric layer in the CMP process starts detecting at t1 and stops detecting at t2. Using the theorem of window logic, the change in the slope of the curve within a fixed window 19 at every time point within the time period xcex94t2 is detected. If the change of the slope of the curve is larger than a predetermined value within the window 19, an output signal is generated to stop the CMP process. Alternatively, if the change of the slope of the curve is always smaller than the predetermined value, the CMP process will automatically stop at the end-point time t2.
The prior art method does not detect the performance of the CMP process within the time period xcex94t1, which is before the time point t1. When errors in the CMP process occur within the time period xcex94t1, they go undetected and an incorrect CMP process will continue to be performed on the semiconductor wafer. This may cause irreversible damage such that the semiconductor wafer must be discarded. This obviously affects the yield of the entire process.
It is therefore a primary objective of the present invention to provide a method for monitoring a semiconductor wafer in a chemical mechanical polishing process to solve the above mentioned problem.
In a preferred embodiment, the present invention provides a monitoring method for monitoring a semiconductor wafer in a chemical mechanical polishing (CMP) process, the CMP process being used to polish a dielectric layer of the semiconductor, the monitoring method comprising:
exposing the dielectric layer of the semiconductor wafer with an input light beam of fixed wavelength at a predetermined angle to generate a reflected light beam within a predetermined time period after performing the CMP process, the intensity of the reflected light beam undergoing periodic changes in response to the gradual thinning of the dielectric layer during the CMP process;
monitoring the intensity of the reflected light beam at a starting period within the predetermined time period and obtaining a periodic change rule according to the periodic changes of the intensity of the reflected light beam; and
monitoring the intensity of the reflected light beam throughout the rest of the predetermined time period and generating an output signal if the change of the intensity of the reflected light beam is not in accordance with the periodic change rule.
It is an advantage of the present invention that the method for monitoring a semiconductor wafer in a chemical mechanical polishing process can detect the performance of the CMP process of a dielectric layer and prevent incorrect CMP processes, thereby improving productivity.